[Xen-devel] [patch 26/44] xen: ignore RW mapping of RO pages in pagetable_init

[Jeremy Fitzhardinge <jeremy@xxxxxxxxxxxxx>]

When setting up the initial pagetable, which includes mappings of all
low physical memory, ignore a mapping which tries to set the RW bit on
an RO pte.  An RO pte indicates a page which is part of the current
pagetable, and so it cannot be allowed to become RW.

Once xen_pagetable_setup_done is called, set_pte reverts to its normal
behaviour.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xxxxxxxxxxxxx>
Acked-by: Chris Wright <chrisw@xxxxxxxxxxxx>
Cc: ebiederm@xxxxxxxxxxxx (Eric W. Biederman)

---
 arch/i386/xen/enlighten.c |   27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

===================================================================
--- a/arch/i386/xen/enlighten.c
+++ b/arch/i386/xen/enlighten.c
@@ -501,7 +501,7 @@ static void xen_write_cr3(unsigned long 
 
 /* Early in boot, while setting up the initial pagetable, assume
    everything is pinned. */
-static void xen_alloc_pt_init(struct mm_struct *mm, u32 pfn)
+static __init void xen_alloc_pt_init(struct mm_struct *mm, u32 pfn)
 {
        BUG_ON(mem_map);        /* should only be used early */
        make_lowmem_page_readonly(__va(PFN_PHYS(pfn)));
@@ -553,9 +553,31 @@ static void *xen_kmap_atomic_pte(struct 
 }
 #endif
 
+static __init pte_t mask_rw_pte(pte_t *ptep, pte_t pte)
+{
+       /* If there's an existing pte, then don't allow _PAGE_RW to be set */
+       if (pte_val_ma(*ptep) & _PAGE_PRESENT)
+               pte = __pte_ma(((pte_val_ma(*ptep) & _PAGE_RW) | ~_PAGE_RW) &
+                              pte_val_ma(pte));
+
+       return pte;
+}
+
+/* Init-time set_pte while constructing initial pagetables, which
+   doesn't allow RO pagetable pages to be remapped RW */
+static __init void xen_set_pte_init(pte_t *ptep, pte_t pte)
+{
+       pte = mask_rw_pte(ptep, pte);
+
+       xen_set_pte(ptep, pte);
+}
+
 static __init void xen_pagetable_setup_start(pgd_t *base)
 {
        pgd_t *xen_pgd = (pgd_t *)xen_start_info->pt_base;
+
+       /* special set_pte for pagetable initialization */
+       paravirt_ops.set_pte = xen_set_pte_init;
 
        init_mm.pgd = base;
        /*
@@ -603,6 +625,7 @@ static __init void xen_pagetable_setup_d
        /* This will work as long as patching hasn't happened yet
           (which it hasn't) */
        paravirt_ops.alloc_pt = xen_alloc_pt;
+       paravirt_ops.set_pte = xen_set_pte;
 
        if (!xen_feature(XENFEAT_auto_translated_physmap)) {
                /*
@@ -741,7 +764,7 @@ static const struct paravirt_ops xen_par
        .kmap_atomic_pte = xen_kmap_atomic_pte,
 #endif
 
-       .set_pte = xen_set_pte,
+       .set_pte = NULL,        /* see xen_pagetable_setup_* */
        .set_pte_at = xen_set_pte_at,
        .set_pmd = xen_set_pmd,
 

-- 


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